This guide tells how to manage low-temperature, in-bin drying of shelled corn. Low-temperature drying is natural air drying (using only a fan) or natural air drying plus supplemental heat to raise the air temperature an additional 2 to 4 degrees Fahrenheit. Natural air drying uses the heat in the outside air plus the heat released from the fan motor, which raises the air temperature about 2 degrees Fahrenheit. The supplemental heat may be provided by gas heaters, electric heaters, solar collectors, etc.

Low-temperature drying has two major advantages: energy efficiency and high quality dried grain.

You can manage low-temperature drying as layer drying, controlled filling, or a combination of the two. This guide addresses all three options.

This guide helps manage grain drying by recommending the depth of wet grain to be added to the bin for various combinations of bin diameter and fan sizes. The depth of grain recommended gives the same air flow per bushel of wet grain for all the bin-fan combinations. Therefore, all combinations have the same probability of drying without mold growth.

Weather records in southwest, central and northern Missouri have been analyzed, and the recommended depths should allow successful drying nine years out of 10. Given the established depth of fill, this guide helps estimate the time required to dry the wet grain and suggests how the drying should be managed. Variation in weather and fan characteristics can change the time required to dry the grain.

If you have difficulty determining how to manage your low-temperature drying from this guide, contact your area agricultural engineering specialist at your local MU Extension center for help.

Layer drying

With low-temperature layer drying, the bin is filled in two or more fillings or layers and each fill layer is dried before the next layer is added. The depth of each fill depends on the moisture content of the corn, the depth of dried corn already in the bin and the amount of air delivered by the fans. Adding layers at depths greater than those recommended increases the chance of mold.

Because the time to dry a layer of wet grain can take four to 14 days, layer drying works best where several bins are equipped for low-temperature drying. Harvesting can proceed more rapidly because the grain is drying in several bins at the same time.

Fill depth recommendations

Figures 1, 2 and 3 give recommended depth of fill for different depths of dried corn already in the bin and for different moisture contents of corn in the fill layer. The depth of fill from Figures 1, 2 or 3 multiplied by the fill adjustment factor in Table 1 gives the depth of fill for the particular fan (or fans) and bin diameter used. If you are selecting a fan (or fans) for your bin, the combinations of fan(s) and bin diameters where the fan factor is bolded in Table 1 are the most economical.

Figure 1Depth of fill and drying time for natural-air drying. The only heat comes from the fan and fan motor, which increase the air temperature about 2 degrees Fahrenheit.

Figure 2Depth of fill and drying time for air heated 2 degrees Fahrenheit by supplemental heater, above natural-air drying.

Figure 3Depth of fill and drying time for air heated 4 degrees Fahrenheit by supplemental heater, above natural-air drying.

Table 1Depth adjustment factor* for fan(s)**

Bin diameter

Fans

One 3 Hp

One 5 Hp

Two 5 Hp

One 7-1/2 Hp

Two 7-1/2 Hp

Three 7-1/2 Hp

One 10 Hp

Two 10 Hp

Three 10 Hp

18 feet

1.00***

1.25

21 feet

0.80

1.15

1.35

1.25

24 feet

0.65

1.00

1.30

1.15

27 feet

0.85

1.20

1.00

1.35

1.20

30 feet

0.85

1.30

1.40

1.05

1.40

33 feet

0.95

1.35

1.50

36 feet

1.05

1.25

0.80

1.25

1.40

*Multiply depth from Figures 1, 2 and 3 by these factors.**This table was prepared on performance data of typical vaneaxial fans. At the grain depths used in drying as described in this guide, vaneaxial fans are recommended.***The adjustment factor is bolded for the fan-bin combination recommended when the unit is purchased.

After the depth of fill has been determined, the estimated time to dry the layer is shown in Figures 1, 2 or 3. This is an estimate. The next layer can be added when a visual inspection or a moisture test determines that the top surface of the last layer added has dried to 15 percent moisture or less.

Steps for management of layer drying system

Find the approximate depth of the initial fill from Figures 1, 2 or 3. The depth is based on the moisture content of the corn put in the bin (fill moisture or Mf) and zero depth of dried corn. Also, find the approximate days required to dry the corn that is given on the moisture content line.

Adjust the depth found in Step 1 by multiplying it by the factor in Table 1 for the fan-bin diameter combination.

Fill the bin to the depth determined in Step 2, and run the fan 24 hours a day until the top of the fill has dried to 15 percent moisture. Use the days-to-dry figure found in Step 1 as a guide.

The approximate depth of the next fill is based on the depth of the dried corn in the bin and the moisture content of the incoming corn (Mf). The depth of the dried grain is less than the fill depth because it shrinks during the drying. The depth of fill after drying is determined by multiplying the depth of wet grain fill by the fraction-of-volume figure in Table 2.

This procedure can be simplified by working with wet corn depths only. The chance of mold will not be increased. Next, find the depth of next fill and the approximate drying time on the moisture content line in Figures 1, 2 or 3.

Table 2Volume shrink of shelled corn dried to 15 percent moisture

Original moisture

Volume at 15 percent as fraction of original volume

30 percent

0.74

28 percent

0.76

26 percent

0.79

24 percent

0.82

22 percent

0.86

20 percent

0.90

18 percent

0.94

16 percent

0.98

15 percent

1.00

Adjust the depth of the corn added to match the size of the fan(s) selected according to the appropriate factor in Table 1.

Make the next fill and run the fan 24 hours a day until the top layer is dried to 15 percent moisture. Use the approximate time from Step 4 as a guide.

If further fills are needed, repeat Steps 4, 5 and 6.

When the top surface of the last fill has dried to 15 percent moisture, run the fan during daytime hours only until the corn is dried to 14 percent moisture.

Low-temperature layer drying does present harvesting limitations at moisture contents above 24 percent. Several fills are required and you must wait for the last fill to dry before adding another layer. If the cost of field losses are combined with the cost of drying, the least costly drying calls for the initial fill to average about 24 percent moisture. The moisture content of the corn in the field is decreasing while the initial fill dries, and it is often low enough so that the bin can be completely filled with the second filling. Harvesting usually is completed in about three weeks.

The following three examples illustrate how to use this guide to manage an in-bin, low-temperature layer drying system for shelled corn.

Add 8.3 feet of corn and run fan 24 hours a day until top surface of corn is 15 percent. Takes approximately 11 days.

Second fill shrinks to 7.5 feet (8.3 feet x 0.90 = 7.5 feet). Dried depth of corn is now 4.9 + 7.5 = 12.4 feet. Corn now harvested tests 18 percent. From Figure 1 (with Mf = 18 percent, and 12.4 feet dried grain depth), the bin can be filled to more than 16 feet, the maximum depth for this bin. Fill bin to 16 feet and run fan 24 hours per day until the top surface of corn is 15 percent moisture.

Run fan during daytime hours until the corn is dried to 14 percent moisture or the moisture required for safe storage.

Figure 4

Figure 5

Figure 6

Figure 7

Example 2

Given

30-foot bin diameter

Mf = 28 percent moisture

Two 7.5-horsepower fans

Air temperature is increased 2 degrees Fahrenheit above natural air by solar collector.

Adjusted depth of fill is 8.0 feet x 0.80 = 6.4 feet. This gives a depth of greater than 16 feet, the maximum depth for this bin.

Fill bin to 16 feet and run for 24 hours a day until top surface of corn is 15 percent or the moisture required for safe storage.

Run fan during daylight hours only until corn is dried to 14 percent moisture. You may have to turn off gas burner to keep from over-drying.

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Controlled filling

Fills can be made before the top surface of a previous fill dries to 15 percent moisture if you carefully control the depth of wet corn above the dried corn. The total depth of wet corn permitted depends on the moisture content of the undried corn in the bin.

Steps for filling while undried corn from a previous fill is in the bin are:

Measure the depth of the wet corn in the bin. Find the depth of the wet corn by probing. A concrete reinforcing rod makes a suitable probe. As long as the rod is being pushed through wet corn, there is considerable resistance. When the rod reaches dry corn, you can push it easily and it seems to break through.

Add wet corn until undried corn has a total depth not greater than the values given in Table 3.

Table 3Guide for controlled filling

Corn moisture

Maximum total depth* of wet corn

18 percent

12 feet

20 percent

8 feet

22 percent

6 feet

24 percent

5 feet

26 percent

4 feet

28 percent

3-1/2 feet

30 percent

2-1/2 feet

*Adjust these depths by multiplying by the appropriate depth adjustment factor for the fan(s) (Table 1).

NoteNever add wet corn with a moisture content higher than that previously placed in the bin unless the surface corn of the previous fill has dried to 15 percent moisture. The next example illustrates how you manage controlled filling.

Adjusted fill depth is 6 feet because the depth adjustment factor is 1.0. The corn will dry about 6 feet divided by 9 days or 2/3 feet per day or 2 feet every 3 days.

The filling schedule to maintain 5 feet of undried grain is listed in Table 4.

Table 4Filling schedule

Day

Feet

1

6 (initial fill depth)

4

1 (brings wet depth to 5 feet)

7

2

10

2

13

2

16

2

19

2

22

Fill to 16 feet to make up for shrinkage

The actual filling schedule should be faster than shown here because the corn moisture in the field is going down while you fill the bin. Add corn at any time to bring the depth of wet corn up to 5 feet when probing shows that the drying front is less than 5 feet from the surface.

Combination layer drying and controlled filling

You can combine layer drying and controlled filling if such a procedure fits your harvesting schedule better. For example, after you place two or three layers in the bin with layer drying, it may be desirable to finish filling the bin before the surface of the top layer is dry. You can add corn if you follow the procedure for controlled filling. In fact, you can switch from one procedure to the other at any time during harvest if you follow the recommendations of this guide for whatever procedure you use.

Management of stored grain

Keep in mind that properly dried grain can go out of condition in storage. It is a valuable possession; protect it.

Store at a moisture content of 14 percent or less.

Level the top surface of the grain.

Aerate in the fall to cool the grain to 40 degrees Fahrenheit. Do not freeze.

Check and record grain temperature every 21 days. Aerate as soon as any increase in temperature is evident.

Warm grain to 65 degrees Fahrenheit in the spring by running aeration fan.

Try to maintain less than 20 degrees Fahrenheit difference between average outdoor temperature and grain temperature.

Check for insects in the fall, spring and summer. If treatment is necessary, use proper procedures.